The cold air return on my forced air furnace is too loud. While it is is tolerable, there is a noticeable sound of rushing air when the furnace is running (not a whistling sound, just a blowing sound).
I have a KeepRite G9MVE (80,000BTU 1600CFM).
I have read a couple of other posts an online resources, which generally seem to suggest:
For #2, I have removed the return air grill, but no change.
For #3, I removed the blower door and ran the furnace again. The return air was not noticeably quieter.
Using some of the calculations in the third link above, I think the 24"x6"=144sq.in trunk is probably too small (something like 900fpm flow), but then why would it not be significantly better with the blower door off? Doesn't this effectively bypass the return air duct?
Is there something I am missing here? How can I improve this issue?
Here is the cold air return intake (A in diagram - 12"x30"=360sq.in) The intake runs between two pairs of floor joists, and the gap closest to the furnace, left side in photo, is noticeably louder than the other side.